The method for producing an unsaturated aldehyde and an unsaturated carboxylic acid through gas-phase catalytic oxidation of propylene, isobutylene, or tertiary butanol with molecular oxygen in the presence of a catalyst has been widely known and also has been industrially used. As the catalyst, for example, the one containing a complex oxide including molybdenum, bismuth and iron as essential components is used (Patent Documents 1 to 6). In this case, the reaction is carried out using the catalyst in a fixed-bed at a temperature of 300 to 400° C.
Such a catalyst which is used in a gas-phase catalytic oxidation reaction is used for a relatively long period of time, and usually, activity of the catalyst is lowered and hence a rate of reaction of a raw material is lowered with time. Usually, in a fixed-bed reactor, reaction temperature is raised to an allowable temperature of a process to maintain the rate of reaction of the raw material in response to the activity-lowering with time (Patent Document 7 and Non-Patent Document 1).    Patent Document 1: Japanese Patent Application Laid-Open No. Sho 53-19,188    Patent Document 2: Japanese Patent Application Laid-Open No. Sho 54-66,610    Patent Document 3: Japanese Patent Application Laid-Open No. Sho 55-359    Patent Document 4: Japanese Patent Application Laid-Open No. Sho 55-19,227    Patent Document 5: Japanese Patent Application Laid-Open No. Sho 56-95,135    Patent Document 6: Japanese Patent Application Laid-Open No. Sho 60-28,824    Patent Document 7: Japanese Patent Application Laid-Open No. Hei 11-263,739    Non-Patent Document 1: Yuichi Murakami ed., “Mechanism of Catalyst Degradation and Prevention Method”, p. 17, Technical Information Institute Co., Ltd.